The invention relates to a shutter drive safety device for a camera, and more particularly, to a drive safety device for a shutter of the type including a shutter opening member and a shutter closing member, each of which is independently operable so that an exposure period is controlled by a time interval between the times of initiation of operation of both the members.
As is well recognized, a shutter of the kind described for a camera is utilized as a focal plane shutter, a lens shutter of two ring type including a shutter opening ring and a shutter closing ring, a shutter of guillotine type including a pair of blades and the like. A focal plane shutter is most frequently used in a single lens reflex camera.
Referring to FIG. 1 for describing one form of a drive control mechanism used with a focal plane shutter which may be used in a single lens reflex camera, it is to be understood that parts other than the drive control mechanism are omitted from illustration. In this Figure, a first blind 1, which represents a shutter opening member, has its right-hand edge disposed around a first blind take-up drum (not shown) while its left-hand edge has an end piece 2 fixedly connected thereto for defining a slit which is used for the purpose of exposure. A pair of first blind pulling strings 3, 4 have their one end anchored to the upper and the lower end of the end piece 2, and extend around a pair of guide pulleys 5, 6 to be wound around the upper and the lower end of a first blind winding drum 7. The pulleys 5, 6 are rotatably fitted on the upper and the lower end of a support shaft 15 associated with a second blind winding drum 9.
A second blind 8, which represents a shutter closing member, has its left-hand portion disposed around the second blind winding drum 9 while its right-hand edge has an end piece 10 secured thereto for defining a slit which is used for the purpose of exposure. A pair of second blind pulling strings 11, 12 have their one end anchored to the upper and the lower end of the end piece 10, and have their other end extending around guide pulleys, not shown, to be disposed around a second blind take-up drum. While not shown, the first blind take-up drum and the second blind take-up drum are urged by shutter charging springs, in the form of strong coiled springs which are internally received therein, in a direction indicated by an arrow a, which represents the shutter blind take-up direction during a film winding operation, all in a manner well known in the art. Both winding drums 7, 9 are fixedly mounted on support shafts 14, 15, respectively, which are rotatably disposed. A first blind pinion 16 and a second blind pinion 17 are integrally mounted on the lower portion of the shafts 14, 15, respectively.
The pinions 16, 17 are in meshing engagement with a first blind winding gear 18 and a second blind winding gear 19, respectively. These gears have substantially similar configuration and diameter, and are rotatably mounted on a common support shaft 13. These gears rotate through one revolution in response to a winding operation of a shutter blind winding shaft, not shown, which rotates during a film winding operation. In this manner, these gears cause both pinions 16, 17 to rotate, thus causing a rotation of the winding drums 7, 9 to wind up the first shutter blind 1 and the second shutter blind 8, respectively. A cam plate 20 is integrally secured to the top surface of the gear 18, and includes a step 20a which operates to prevent a rotation of the gear 18 in the reverse direction as the latter has been rotated through a given angle.
The step 20a in the cam plate 20 is engaged by a hook 21a formed on one end of a first blind detent lever 21 which is pivotally mounted on a pin 23, secured to a stationary member, substantially centrally along its length. The other end of the detent lever 21 is formed with a folded piece 21b which depends downwardly from the lateral edge. A torsion spring 25 is disposed on the pin 23 and has its one end engaged with a lateral edge of the lever 21 toward the hook 21a and its other end engaged with an anchorage pin 24, thus urging the lever 21 to rotate counter-clockwise. However, the resulting rotation is limited by the abutment of the hook 21a against the peripheral edge of the cam plate 20.
A peg 31 is fixedly mounted on the top surface of the gear 19 adjacent to the periphery thereof for engagement by a locking end 26a of a second blind detent lever 26 at the termination of a second blind winding operation, thus preventing a reverse rotation of the gear 19. This winding operation takes place by a rotation of the gear 19 through a given angle. The detent lever 26 is L-shaped, and its bend is pivotally mounted on a pin 27. The other end of the lever 26 is formed with a folded piece 26b which depends downwardly in a direction perpendicular thereto from the lateral edge thereof. A release electromagnet 30 has its armature 30a disposed in opposing relationship with the folded piece 26b. A torsion spring 29 is disposed on the pin 27 and has its one end engaged with one arm of the lever 26 and its other end engaged with an anchorage pin 28, thus urging the lever 26 to rotate counter-clockwise about the pin 27. However, the folded piece 26b is normally held attracted by the armature 30a of the electromagnet 30, thus suppressing the rotation of the lever. An upright lug 32 is fixedly mounted on the top surface of the gear 18 adjacent to its periphery for engagement with another lug 33 fixedly mounted on the bottom surface of the gear 19 adjacent to its periphery and depending downwardly therefrom. During a film winding operation, the lugs 32, 33 engage with each other to connect both the winding gears 18, 19 integrally. Another lug 34 is fixedly mounted on the bottom surface of the gear 18 adjacent to its periphery and depends downwardly therefrom. During the operation of the first blind 1 or the shutter, the lug 34 is adapted to abut against a stop pin 35, fixedly mounted on a stationary member, thus ceasing the rotation of the gear 18 after it has rotated counter-clockwise through substantially one revolution.
In operation, FIG. 1 illustrates a shutter charged condition when the first and the second blind 1, 8 have been wound up. Specifically, the both gears 18, 19 have clockwise rotated through one revolution, respectively, in response to the operation of a film winding mechanism, not shown. As a result, both the pinions 16, 17 have caused both the drums 7, 9 to rotate counter-clockwise, and both the gears 18, 19 are maintained in locked condition by the respective detent levers 21, 26 even though the drums are urged in a direction indicated by the arrow a, thus achieving a shutter charged condition.
Subsequently, as the folded piece 21b is pushed in a direction indicated by an arrow b in response to the depression of a shutter release button, not shown, the lever 21 rotates clockwise about the pin 23, whereby the hook 21a is disengaged from the step 20a on the cam plate 20. Thereupon, the first blind winding gear 18 which is integral with the cam plate 20 is allowed to rotate counter-clockwise rapidly under the resilience of the charging spring, thus causing the first blind 1 to run in the direction of the arrow a at a high rate. When the winding gear 18 has rotated through substantially one revolution until lug 34 abuts against the stop pin 35, it ceases to rotate. On the other hand, the second blind winding gear 19 is prevented from rotating since the peg 31 abuts against the locking end 26a of the detent lever 26 at the time the first blind winding gear 18 begins its rotation, thus preventing the second blind 8 from running while the first blind 1 is allowed to run alone.
At the same time as the shutter release occurs, a movable reflecting mirror (not shown) moves upward to initiate an exposure process, whereby light reflected from the first blind and the film surface (not shown) impinges upon a light receiving element, not shown, for the purpose of photometry. When a proper amount of light has been received, an exposure control circuit produces an output which deenergizes the electromagnet 30. Thereupon, the armature 30a is no longer able to hold the folded piece 26b attracted thereto, whereby the second blind detent lever 26 is allowed to rotate counter-clockwise under the bias applied thereto. Consequently, the locking end 26a is disengaged from the peg 31, allowing the second blind winding gear 19 to rotate rapidly counter-clockwise under the resilience of the charging spring to thereby allow the second blind 8 to run. When the gear 19 rotates through substantially one revolution the lug 33 thereon abuts against the lug 32 on the first blind winding gear 18 which remains at rest after having completed its rotation through substantially one revolution. In this manner, the gear 19 ceases to rotate, thus terminating the running of the second blind 8. In the manner mentioned above, the first blind 1 and the second blind 8 begin their running with a time interval therebetween which is controlled by an exposure control circuit to thereby open and close an exposure light path. Light from an object being photographed which passes through a taking lens reaches a film surface, thus allowing the image of the object to be formed on the film surface with a proper amount of light.
After the completion of a photographing operation, a film winding operation causes both the first blind 1 and the second blind 8 to be wound up on their winding drums, thus achieving a shutter charged condition. During such winding operation, the end pieces 2, 10 on the both blinds 1, 8 are maintained in overlapping relationship, as shown, in order to prevent a leakage of light through a clearance formed between the blinds 1, 8. The blinds are maintained in their wound position by respective detent levers 21, 26 while maintaining such overlapping relationship. The arrangement is now ready to initiate another shutter operation.
In the operation of the shutter drive control mechanism mentioned above, it will be seen that for an exposure period less than 1/1000 second, there will be a very short time interval from the unlocking of the first blind winding gear 18 from the detent lever 21 to the unlocking of the second blind winding gear 19 from the detent lever 26. For this reason, it is necessary that the unlocking of the gear 19 from the detent lever 26 be achieved in a smooth manner. However, the adjustment is very troublesome, and the arrangement is liable to cause an in proper operation of the exposure function because of the unsmooth operation. Also, while the arrangement may operate smoothly under normal temperatures, the operation may be rendered unsmooth under reduced temperatures. An added difficulty is experienced if it is attempted to assure a smooth operation under all temperatures. If the resilience of the spring 29 is increased in an attempt to facilitate the disengagement of the detent lever 26 from the peg 31 on the gear 19, oscillations and shocks occurring during a transportation of a camera or other factors may readily cause the disengagement, allowing the both blinds 1, 8 to begin running simultaneously independently from the functioning of an exposure control circuit, resulting in a failure to take a picture.